1. Field of the Invention
The present invention relates to an optical system used in an optical read/write device for reading or writing information on optical discs employing different wavelengths. More particularly, it relates to an optical pickup that provides compatibility with DVDs and HD-DVDs, which employ laser sources of different wavelengths.
2. Description of the Related Art
Optical read/write devices include an optical disc player that can read information recorded on optical discs or optical recording media such as DVDs (Digital Versatile Discs). Some DVDs come in 4.7 GB but highly denser package media are demanded and under development. As well known, it is effective to provide the package media with a light source of shorter wavelength and an objective lens of a higher NA (numeral aperture) to improve recording density. As for the light source of a shorter wavelength, short wavelength semiconductor lasers with a GaN substrate employed as the base are being developed and nearing practical use. The laser under development has a wavelength of 405 nm and a high density DVD (HD-DVD) system, employing the laser, with about 15 GB is also being developed.
Accordingly, these DVDs and HD-DVDs require a compatible disc player that can read information recorded thereon, and the reproduction system thereof should be able to reproduce DVDs. What is raised as a problem here is that a laser of a short wavelength cannot read a two-layered disc among DVD discs. This is resulted from the fact that the intermediate layer of the two-layered disc has a low reflection coefficient for a beam of light of a short wavelength. Accordingly, to implement a compatible disc player, it is necessary to provide the HD-DVD system with a laser that emits a red beam (hereinafter also simply referred to as red) with a wavelength of around 650 nm in addition to a blue beam (hereinafter also simply referred to as blue) with a wavelength of around 405 nm. Conventional DVDs have a substrate 0.6 mm in thickness, a wavelength of from 635 nm to 655 nm, and the numerical aperture of an objective lens of about 0.6, while HD-DVDs have a substrate 0.6 mm in thickness, a wavelength of 405 nm, and the numerical aperture of an objective lens of about 0.6.
However, it is difficult to condense both beams of light having different wavelengths by means of conventional single lenses without substantial aberration because of chromatic aberration of the objective lens. Thus, this requires some thought to ensure compatibility between DVDs and HD-DVD.
It is conceivable to switch between dedicated objective lenses for each of the wavelengths in use in order to implement an optical pickup for a compatible player of DVDs and HD-DVDs. However, this requires two objective lenses and therefore a complicated lens switching mechanism. This increases cost and the actuator in size, providing a disadvantage to a reduction in size. Alternatively, the objective lens and the collimator lens can be conceivably combined with each other. However, this configuration would make it difficult to maintain the performance for displacing the objective lens since the collimator is fixed relative to the objective lens.
In either case, dedicated optical systems, which are provided with prisms or lenses and a plurality of light sources to ensure compatibility between DVDs and HD-DVDs, would tend to result in a complicated and large optical pickup or entire optical head.
The present invention was developed in view of the aforementioned problems. An object of the present invention is to provide an optical pickup that is suitable for a decrease in size and can read and write information on optical discs or recording surfaces available for different wavelengths.
The optical pickup according to the present invention comprises a first light source for emitting a first beam of light with a first wavelength and a second light source for emitting a second beam of light with a second wavelength, longer than the first wavelength. The optical pickup also comprises a condenser lens for condensing the aforementioned first and second beam of light onto an information recording surface of a recording medium; and a diffractive optical element having a corrugation arranged in an optical path from the aforementioned first and second light source to the aforementioned condenser lens. The optical pickup is characterized in that the aforementioned condenser lens condenses, for the aforementioned first beam of light, a refracted beam of the first diffraction order of the first beam of light by means of the aforementioned diffractive optical element as an information read beam or an information write beam. In addition, for the aforementioned second beam of light, a refracted beam of the second diffraction order, lower than the aforementioned first diffraction order, of the second beam of light is condensed by means of the aforementioned diffractive optical element as an information read beam or an information write beam.
The optical pickup according to the present invention is characterized in that the aforementioned diffractive optical element has the shape of saw teeth in cross section of the corrugation.
Alternatively the optical pickup according to the present invention is characterized in that the aforementioned diffractive optical element has the shape of steps in cross section of the corrugation.
The optical pickup according to the present invention is characterized in that an absolute value of the aforementioned first diffraction order of the diffracted beam of the aforementioned first beam of light is greater by one than an absolute value of the aforementioned second diffraction order of the diffracted beam of the aforementioned second beam of light, and the absolute value of the aforementioned second diffraction order of the diffracted beam of the aforementioned second beam of light is not less than one.
The optical pickup according to the present invention is characterized in that the diffracted beam of the aforementioned second beam of light is a primary diffracted beam when the diffracted beam of the aforementioned first beam of light is a secondary diffracted beam, or the diffracted beam of the aforementioned second beam of light is a secondary diffracted beam when the diffracted beam of the aforementioned first beam of light is a tertiary diffracted beam.
The optical pickup according to the present invention is characterized in that a depth of the corrugation of the aforementioned diffractive optical element lies within a range of 1.42 micrometers +/xe2x88x920.2 micrometers or 2.40 micrometers +/xe2x88x920.2 micrometers.
The optical pickup according to the present invention is characterized in that a pitch of the corrugation of the aforementioned diffractive optical element is 20 micrometers or greater.
The optical pickup according to the present invention is characterized in that the aforementioned first wavelength lies in a range of from 400 nm to 410 nm and the aforementioned second wavelength lies in a range of from 630 nm to 660 nm.
The optical pickup according to the present invention is characterized in that the aforementioned diffractive optical element has a plano-concave lens and the aforementioned diffractive optical element is formed on the concave surface of the plano-concave lens.
The optical pickup according to the present invention is characterized in that the aforementioned diffractive optical element is formed integrally with the aforementioned condenser lens and the corrugation of the aforementioned diffractive optical element is formed on a surface of the aforementioned condenser lens, the surface being oriented towards the light sources.